Ultrasonic motors

ABSTRACT

An ultrasonic motor is described which uses radial vibrations of an electro-active material disc ( 7 ) amplified by one or more flextensional diaphragms ( 6 ) to drive a rotor ( 4 ) pressed in frictional contact with the diaphragm ( 6 ) by a force imposed by a spring ( 3 ) or magentic attraction. The vibrations are converted by elastic fins ( 5 ) into rotary motion of the rotor ( 4 ). The motor can be operated in any resonant mode that generates vibration at the surface perpendicular to the contact area. Versions of the motor with one or two rotors are disclosed with the two rotor version being used to produce an output in the same direction or opposite directions.

[0001] This invention concerns improvements in or relating to ultrasonicmotors.

[0002] Ultrasonic motors produce high torque with a low maximum speedand have a high power and torque density. There are a large number ofcurrent designs but almost all designs require precision constructionmethods and so are expensive to make.

[0003] The present invention is intended to remedy this drawback. Moreparticularly, the present invention seeks to provide an ultrasonic motorthat can be made simply, with few components and at low cost.

[0004] According to the broadest aspect of the present invention thereis provided an ultrasonic motor in which oscillating vibrations areconverted into rotary motion through frictional contact at an interfacebetween relatively rotatable components of the motor wherein one of thecomponents comprises a disc of electro-active material and at least oneflextensional displacement amplifier diaphragm for converting radialvibrations of the disc into oscillating vibrations of the or eachdiaphragm perpendicular to the plane of the disc.

[0005] The invented ultrasonic motor utilises one or two flextensionaldiaphragms as displacement amplifiers attached to the disc. Thesegreatly increase the amplitude of vibrations and convert the radialvibrations of the disc into vibrations of the or each diaphragmperpendicular to the disc plane. Such flextensional amplifiers are usedfor sonar applications and for increasing the displacement given byelectro-active displacement transducers. The innovative aspect of thisdesign is that it converts the vibrations to rotational movement by africtional push and release method.

[0006] Furthermore, the invented ultrasonic motor requires only a singleelectrical supply phase to drive it and has a positive braking torquewith no power applied. The excitation of the disc in a radial mode givesa high electromechanical coupling factor producing an efficient and highpower density ultrasonic motor. The high coupling factor also allows theuse of a simple ac electrical drive without feedback to controlfrequency since the resonant bandwidth of the motor is large.

[0007] Preferably, the frictional contact is provided by a plurality ofdrive elements at the contact interface. In a preferred arrangement, thedrive elements comprise elastic fins on one of the diaphragm(s) and anopposed rotor which are pressed into frictional contact with the otherof the diaphragm and rotor. For example, the diaphragm and rotor may beurged towards each other by a spring force. Alternatively, the diaphragmand rotator may be urged towards each other by a magnetic attractionforce.

[0008] Furthermore, this method of producing a rotary motion from avibrating surface relies on a frictional contact between the tips of theelastic fins and the relatively moving surface, which is maintained witha positive contact pressure supplied by a spring or magnetic attraction.The fins impart a small displacement on the rotor, of the order of a fewmicrons, on each upward stroke of the diaphragm then the fin retreats toreturn to its original position on the downward travel of thediaphragm's surface.

[0009] Embodiments of the invention will now be described in moredetail, by way of example only, with reference to the accompanyingdrawings, wherein:

[0010]FIG. 1 shows in cross-section a first embodiment of an ultrasonicmoor according to the invention having a single rotor;

[0011]FIG. 2 is a perspective view from above of the motor shown in FIG.1;

[0012]FIG. 3 is a cross-section similar to FIG. 1 showing a modificationto the motor;

[0013]FIG. 4 is a perspective view from above of the motor shown in FIG.4;

[0014]FIG. 5 shows in cross-section a second embodiment of an ultrasonicmotor according to the present invention having two rotors;

[0015]FIG. 6 is a cross-section similar to FIG. 5 showing a modificationto the motor;

[0016]FIG. 7 shows in cross-section a third embodiment of an ultrasonicmotor according to the present invention having two rotors;

[0017]FIGS. 8a and 8 b show schematically operation of the fin shown inFIGS. 1 and 2 for producing rotary motion at the rotor/diaphragminterface on the upstroke and downstroke of the diaphragm;

[0018]FIGS. 9a and 9 b show schematically operation of an alternativeL-shaped fin for producing rotary motion at the rotor/diaphragminterface on the upstroke and downstroke of the diaphragm;

[0019]FIG. 10 shows in cross-section a fourth embodiment of anultrasonic motor according to the present invention having one rotor;and

[0020]FIG. 11 shows in cross-section a fifth embodiment of an ultrasonicmotor according to the present invention having two rotors.

[0021] Where appropriate, in the following description of the exemplaryembodiments of ultrasonic motor according to the present invention, likereference numerals are used to indicate corresponding parts.

[0022] Referring first to FIGS. 1 and 2 of the accompanying drawings, anultrasonic motor is shown having a stator comprising a disc 7 primarilyconstructed from an electro-active material with a respectiveflextensional amplifier diaphragm 6 a,6 b bonded to each face.

[0023] The disc 7 comprises a piezoceramic disc 2 mm thick and 25 mmoutside diameter with a 4 mm internal diameter hole drilled centrallyusing a tungsten carbide drill. The disc 7 is a multi-layer constructionwith one or more layers of electro-active material interleaved withlayers of conductive electrode material and is electroded withapproximately 7 μm of fused silver and poled in the thickness direction.

[0024] The flextensional diaphragms 6 a,6 b are constructed of 0.5 mmdiscs of phosphor bronze PB 102 or 2% beryllium bronze, both lowinternal loss materials, wire electro-discharge machined to 25 mmoutside diameter with a 4 mm internal diameter hole. The discs areshaped into the flextensional shapes with a two piece punch (not shown),turned to form the inside and outside surfaces of each diaphragm 6 a, 6b. A central locating pin (not shown) inserted through each disc alignsit to the forming punch which is loaded with 3 kN to form the diaphragm6 a, 6 b.

[0025] Each flextensional amplifier diaphragm 6 a,6 b has an exteriorflange 8 bonded to the piezoceramic disc 7 with RS Circuit works silverloaded epoxy to make the electrical contact to the disc electrodes. Atthe same time an electrical input wire (not shown) is attached to eachdiaphragm 6 a,6 b with the same epoxy. The whole assembly is then bakedat 60° C. for 10 minutes to cure the epoxy.

[0026] Extending through the aligned central holes in the disc 7 andflextensional amplifier diaphragms 6 a,6 b is an axle 1 providing abearing 10 for a rotor in the form of a disc 4. The axle 1 comprises anM3×30 nylon bolt bonded at 9 to the diaphragm 6 a with adhesive such ascyanoacrylate glue. The disc 4 is arranged opposite the other diaphragm6 b and is urged towards the diaphragm 6 b by a helical spring 3retained on the axle 1 by an M4 nut 2. The nut 2 is threaded on the axle1 whereby the pre-load of the spring 3 can be adjusted.

[0027] The disc 4 has a 12 mm outside diameter with a 4 mm internaldiameter central hole and is constructed from the same material as theflextensional amplifier diaphragms 6 a, 6 b. The disc 4 has integraldrive transmitting means in the form of three elastic fins 5 uniformlyspaced in the circumferential direction and extending at an angle of 30°to the plane of the disc 4 for co-operating with the opposed diaphragm 6b. The fins 5 are formed by cutting and bending the material of the disc4, for example by making wire electro-discharge machined cuts ofL-shape.

[0028] To test the motor, the nut 2 was tightened to preload the spring3 to 5N and the electrical input excited by a single phase, 0-100 kHz,ac supply at 15.7 kHz (fundamental resonant mode of diaphragmtransducer), 48 kHz (2^(nd) mode) and 82 kHz at a voltage of 40 Vrms. Noextra friction layer was added to the planar surface of the stator.

[0029] Operation in the audio range at the fundamental resonant mode wastested, although the sound emission was unpleasant. Therefore, testswere carried out at resonant modes of the transducer with frequencies.Rotation of the rotors occurred within 500 Hz either side of the 48 kHzand 82 kHz resonance modes, corresponding with vibration modes givingorthogonal motion at the rotor/stator contact. The speed of rotationincreased to a peak at the resonance frequency, corresponding toapproximately 150 rpm at the 48 kHz mode and 250 to 300 rpm at the 82kHz mode. Rotation also occurred at approximately 100 rpm at the 15.7kHz mode.

[0030] As will be understood, the operation of the motor to produce arotary motion from a vibrating surface relies on frictional contact atthe interface between the disc 4 and the opposed diaphragm 6 b which ismaintained with a positive contact pressure supplied by the spring 3.More particularly, the fins 5 impart a small displacement on the disc 4,of the order of a few microns, on each upward stroke of the diaphragm 6b, then the fins 5 retreat to return to their original position on thedownward stroke of the diaphragm 6 b.

[0031] This operation is shown schematically in FIGS. 8a and bb. Eachfin acts as a cantilever, bending elastically without sliding on thesurface of the diaphragm due to the friction between the fin tip anddiaphragm surface with the upward motion of the diaphragm, the contactforce being the addition of the fin spring force and the acceleration ofthe rotor upwards. This friction drives the rotor horizontally. Thisside thrust is due to the instantaneous rotation of the tip about apoint above the diaphragm surface, and not in line with the contactpoint in the direction of vibration, hence being dependent on thestructure of the fin. This rotation point moves with the bending of thefin and movement of the rotor and is shown in FIG. 8a at the beginningof the upstroke of the diaphragm. On the downstroke of the diaphragm,the fin relaxes and the tip slides along the diaphragm surface as shownin FIG. 8b. The tip friction on the downstroke is much less than theupstroke since the contact force is due to the acceleration of the rotordeducted from the spring force of the fin

[0032] The provision of two diaphragms, one on each side of the disc 7,with the axle 1 attached to one of the diaphragms effectively doublesthe relative contact vibration at the interface. In a modification(FIGS. 3 and 4), the elastic fins 5 are integrally formed from thematerial of the diaphragm 6 b opposite the disc 4. In anothermodification (not shown), only one diaphragm is provided and the axle 1is attached to the disc 7.

[0033] Referring now to FIG. 5, there is shown a second embodiment ofultrasonic motor in which two discs 4 a,4 b are provided, one oppositeeach diaphragm 6 a,6 b respectively with each disc 4 a,4 b having threeintegral elastic fins 5 a,5 b for co-operating with the associateddiaphragm 6 a, 6 b. The rotor 4 b is bonded at 9 to the axle 1 and thedisc 7 provides a bearing 10 for the axle 1. For this, the disc 7 has anylon bush (not shown) located in the central hole which allows movementof the axle 1 with the oscillating vibrations of the diaphragms 6 a, 6b. The operation of this embodiment is similar to the first embodimentabove-described with oscillating movement of the diaphragms 6 a,6 bbeing converted to rotary motion of the rotors 4 a,4 b at the frictioncontact interfaces therebetween. In a modification (FIG. 6) the elasticfins 5 a,5 b are formed from the material of the diaphragms 6 a, 6 b.

[0034] With reference now to FIG. 7, there is shown a third embodimentof ultrasonic motor in which there are two discs 4 a,4 b, one oppositeeach diaphragm 6 a,6 b with each disc 4 a,4 b having three integralelastic fins 5 a,5 b for co-operating with the associated diaphragm 6 a,6 b. The axle 1 is bonded at 9 to the disc 7 and provides bearings 10for both discs 4 a,4 b which are urged towards the associated diaphragm6 a,6 b by a respective helical spring 3 a,3 b. Each spring 3 a,3 b isretained by a nut 2 a,2 b for independently adjusting the pre-load withthrust bearings (not shown) between the springs 3 a,3 b and theassociated disc 4 a,4 b. With this arrangement, the discs 4 a,4 b can berotated at different speeds and/or in different directions for the samelevel of vibration by altering the angle of the fins 5 a,5 b to thedirection of vibration of the diaphragms 6 a, 6 b. In this way, abi-directional motor may be obtained. In a modification (not shown), thefins 5 a,5 b are provided by the diaphragms 6 a, 6 b.

[0035] Turning now to FIG. 10, there is shown a fourth embodiment ofultrasonic motor in which the thickness of the motor can be considerablyreduced compared to the previous embodiments.

[0036] In this embodiment, a piezoceramic disc 11 is provided comprisingan annular ring-like element having an enlarged central hole 11 a inwhich upset, concave central regions 13 a of flextensional amplifierdiaphragms 13 are received. In this way, the operating region of thediaphragms 13 is contained inside the planes defined by the outer facesof the disc 11 reducing the overall thickness compared to the previousembodiments in which the central regions of the diaphragms are bent awayfrom the disc 11.

[0037] The disc 11 is shown as two rings sandwiched together with ametal shim 12 forming an electrical contact. In thise case, the tworings 11 are glued or otherwise fixed (for example by soldering)together with the metal shim 12 so that their positively (oralternatively negatively) poled faces are in contact with the metalshim.

[0038] A rotating component 14 is pressed in contact with one of thevibrating flextensional amplifier diaphragms 13. This is equipped withangled vanes or legs as described previously such that the vibrations ofthe flextensional amplifier diaphragm 13 causes the component 14 torotate. The rotating component 14 is pressed in contact with theflextensional amplifier diaphragm 13 by a spring in this case shown as aspring clip 18 acting through a bearing 17, the whole assembly beingheld in place by a post arrangement 16 with electrical contact made atthe point 15.

[0039] It will readily be appreciated that any of the modificationsdescribed previously for the other embodiments of this type of motor canbe applied to this embodiment.

[0040] For example, two rotating components 14 can be used, one on eachside of the vibrating disc and pressed against each flextensionalamplifier diaphragm.

[0041] The piezoceramic disc 11 need not be made as two rings fixedtogether with a central shim contact, but can be a single ring with twoouter contacts. Similarly, the piezoceramic disc 11 can be made as aplurality of thin piezoceramic layers with interleaved contacts,designed to vibrate as a single component.

[0042] In another variation (not shown), the components 14 bearing thevanes or legs referred to as “rotors” above can be held fixed, in whichcase the piezoceramic disc 11 will rotate. In this case the electricaldriving power can be delivered by means of carbon bushes or other slipcontacts.

[0043] In a fifth embodiment shown schematically in cross section inFIG. 11 the central post 16 of the previous embodiment is dispensedwith. The two components 14 bearing the vanes are held fixed and pressinto contact with the flextensional amplifier diaphragms 13 by forcesapplied in the direction of the arrows 19. In this case the piezoceramicdisc 11 becomes the rotating component.

[0044] The vanes are arranged to press against angled portions 13 b ofthe amplifiers 13 such that they constrain the rotating piezoceramicdisc 11 to rotate about a central axis perpendicular to the plane of thepiezoceramic disc 11.

[0045] It will be understood that the invention is not limited to theembodiments above-described. Firstly, the stator disc material can beany active material that can produce ultrasonic vibrations by coupling amagnetic or electrical stimulus to mechanical strain, such asmagnetostrictive, electrostrictive or piezoelectric materials. Thediaphragms can be made from any elastic material having the requiredflextensional properties. However, materials with low internal damping,such as the bronzes above-described, are preferred.

[0046] Furthermore, there are variations in the method of attaching theor each diaphragm to the electro-active stator ring or disc. Adhesivebonding, such as the use of epoxies, conductive metal loaded epoxies orphenolic resins, soldering and other material bonding methods can beemployed, depending on choice of electro-active material and elasticdiaphragm material.

[0047] The geometry of the diaphragm and rotor combination can beoptimised for different speed versus torque performance The shape,thickness and dimensions of the diaphragm will change the amount ofamplification of the mechanical vibrations, the resonant frequency andforce factor of the motor. Also, the number and/or inclination of theelastic fins can be altered from that described. Thus, increasing thenumber of fins will reduce contact pressure and wear while increasingthe inclination of the fins will produce higher speeds and wear.

[0048] The elastic fins may have other shapes, which may give betterefficiency, torque and/or power output. For instance an ‘L’ shaped finwith one end of the ‘L’ attached to the rotor or diaphragm and the otherend driving against the friction interface will also give unidirectionalrotation as shown in FIGS. 9a and 9 b depicting the upstroke anddownstroke of the diaphragm respectively. Similarly a curved fin wouldgive rotation. The fins can be any elastic shape with a point which thepoint which contacts the friction interface rotates around when thediaphragm presses against the fin, where the said point of rotation isnot in line with the contact point in the vibration's direction and isabove the diaphragm's surface. This configuration will impart ahorizontal displacement to the rotor once per the diaphragm vibrationcycle. On the upstroke a horizontal friction force urges the rotor roundas the diaphragm pushes the fin tip to the side, whereas on thedownstroke much less force between the fin tip and diaphragm occurs,allowing sliding of the tip along the diaphragm surface.

[0049] The axle design has many parameters that affect operation of themotor. The axle can be free to turn relative to the diaphragm andelectro-active material assembly, with a journal bearing surface in thecentre of the stator disc such as the motors shown in FIGS. 5 and 6.Another design has an axle which passes through the flextensionalassembly and is fixed to one diaphragm which adds the vibration of thediaphragms together to increase the speed and power of a single rotorwhich rotates about the axle contacting the free diaphragm such as themotors shown in FIGS. 1 to 4. Yet another design has the axle attachedto the electro-active stator disc, with thrust bearings between eachrotor's spring and the respective rotor such as the motor shown in FIG.7.

[0050] The method of applying contact pressure between the rotors anddiaphragms includes alternatives to the motor construction withreference to the spring assemblies shown. Firstly, the springs can be ofmany types apart from the helical coils shown, for instance dish orcrinkle washers can be used to decrease the axial length of the springassembly. Moreover, the springs can be removed by creating magneticattraction between diaphragms and rotors by fabricating one of the setof components from a magnetic material, such as a ferrite, with aremnant magnetic polarisation, and the other set of components from aferromagnetic material such as another remnant magnetic material, iron,nickel or cobalt or their alloys. Another variant uses an electromagnetto provide the magnetic attraction to hold the rotor(s) and diaphragm(s)in contact.

[0051] The rotor/stator contact can also be modified by inserting thinfriction layers of elastic materials on the planar contact surface, suchas polymer layers which are commonly used in present ultrasonic motors,to optimise the output efficiency and power of the motor.

[0052] Finally, an important modification that has potential benefits inthe construction of a kinematic drive line is to hold the aforementioned‘rotors’ stationary and allow the electro-active material anddisplacement amplifier diaphragm assembly to be the moving entity.

[0053] Other variations and modifications will be apparent to thoseskilled in the art and the invention is not intended to be limited tothe specific embodiments above-described.

[0054] The invented ultrasonic motor has particular, but not exclusiveapplication, where a motor which holds its position in the absence ofsupplied power would be desirable, for example toys, positioningapplications such as central heating valves cameras (lens focusing, filmwinding—especially disposable cameras), electric windows etc.

1. An ultrasonic motor in which radial vibrations of a disc ofelectro-active material are converted via at least one flextensionaldisplacement amplifier diaphragm into vibrations of the or eachdiaphragm perpendicular to the plane of the disc, said diaphragmvibrations then being converted into rotary motion via frictionalcontact at a diaphragm/rotor interface.
 2. An ultrasonic motor asclaimed in claim 1 wherein the disc of electro-active material is apiezoelectric material, with an electrode of a conductive materialdeposited on each circular face of the disc.
 3. An ultrasonic motor asclaimed in 1 wherein the disc of electro-active material is anelectrostrictive material, with an electrode of a conductive materialdeposited on each circular face of the disc.
 4. An ultrasonic motor asclaimed in 1 wherein the disc of electro-active material is amagnetostrictive material excited by an oscillating magnetic field. 5.An ultrasonic motor as claimed in any preceding claim wherein the discof electro-active material is of a multi-layer construction with one ormore layers of electro-active material interleaved with layers ofconductive electrode material.
 6. An ultrasonic motor as claimed in anypreceding claim wherein the or each flextensional displacement amplifierdiaphragm is bonded to the surface of the electro-active disc with anepoxy or a metal loaded epoxy.
 7. An ultrasonic motor as claimed in anyone of claims 1 to 5 wherein the or each flextensional displacementamplifier diaphragm is bonded to the surface of the electro-active discwith an anaerobic adhesive or modified anaerobic adhesive.
 8. Anultrasonic motor as claimed in any one of claims 1 to 5 wherein the oreach flextensional displacement amplifier diaphragm is soldered ordiffusion bonded to the surface of the electro-active disc.
 9. Anultrasonic motor as claimed in any preceding claim wherein a respectivediaphragm is attached to each side of the disc and a single rotorpositioned opposite one of the diaphragms turns about an axle which isattached to the other diaphragm.
 10. An ultrasonic motor as claimed inany one of claims 1 to 8 wherein a respective diaphragm is attached toeach side of the disc and a respective rotor is arranged opposite eachdiaphragm of which one rotor is attached to an axle and the other canslide axially along the axle.
 11. An ultrasonic motor as claimed in anyone of claims 1 to 8 wherein an axle is attached to the electro-activematerial disc and one or more rotors turn about said axle on bearings.12. An ultrasonic motor as claimed in any preceding claim wherein one ormore rotors are held in contact with the displacement amplifierdiaphragms' oscillating surfaces utilising magnetic attraction, whenthis magnetic attraction is brought about by the rotors having a remnantmagnetic polarisation and the diaphragms being made of ferromagneticmaterials, such as the metals Nickel, Iron and Cobalt and most alloyscontaining one or more of these metals.
 13. An ultrasonic motor asclaimed in any one of claims 1 to 11 wherein one or more rotors are heldin contact with the displacement amplifier diaphragms' oscillatingsurfaces utilising magnetic attraction, when this magnetic attraction isbrought about by the diaphragms having a remnant magnetic polarisationand the rotors being made of ferromagnetic materials, such as the metalsNickel, Iron and Cobalt and most alloys containing one or more of thesemetals.
 14. An ultrasonic motor as claimed in any one of claims 1 to 11wherein one or more rotors are held in contact with the displacementamplifier diaphragms' oscillating surfaces utilising magneticattraction, when this magnetic attraction is brought about by anelectromagnet winding.
 15. An ultrasonic motor as claimed in any one ofclaims 1 to 11 wherein one or more rotors are held in contact with thediaphragms by one or more springs.
 16. An ultrasonic motor as claimed in1 wherein the displacement amplifier diaphragm and electro-active discassembly is the rotating component and the rotor is the stationarycomponent.
 17. An ultrasonic motor as claimed in 1 wherein thedisplacement amplifier diaphragm and electro-active disc assembly is thestationary component and the rotor is the rotating component.
 18. Anultrasonic motor as claimed in any preceding claim wherein a layer orstructure of an elastic material is attached to the surface of therotor/diaphragm interface.
 19. An ultrasonic motor as claimed in anypreceding claim wherein elastic fins are provided at the interface thateach have a fin tip which contacts the friction interface such that, thefin tip has an instantaneous rotation about a rotation point not in linewith the fin tip contact point in the direction of rotation, thuscausing a horizontal friction reaction which drives the rotor on theexpansive stroke of the displacement amplifier, yet allows the fin torelax on the downstroke and the fin tip to slide on the frictioninterface.
 20. An ultrasonic motor as claimed in claim 19 wherein theelastic fins make a contact at an oblique angle to the surface of thefriction interface between the rotating component and the stationarycomponent.
 21. An ultrasonic motor as claimed in claim 19 or claim 20wherein the elastic fins, which make contact with the frictioninterface, have one or more curved sections in their length.
 22. Anultrasonic motor as claimed in claim 19 or claim 20 wherein the elasticfins, which make contact with the friction interface, have at least twostraight sections that are joined in at an angle.
 23. An ultrasonicmotor in which oscillating vibrations are converted into rotary motionthrough frictional contact at an interface between relatively rotatablecomponents of the motor wherein one of the components comprises a discof electro-active material and at least one flextensional displacementamplifier diaphragm for converting radial vibrations of the disc intooscillating vibrations of the or each diaphragm perpendicular to theplane of the disc.
 24. An ultrasonic motor according to claim 23 whereinthe other component comprises a further disc positioned opposite atleast one diaphragm.
 25. An ultrasonic motor according to claim 24wherein one of the components is stationary and the other component isrotatable relative thereto.
 26. An ultrasonic motor as claimed in anypreceding claim wherein the or each flextensional amplifier diaphragm isdish-shaped with an upset central region.
 27. An ultrasonic motor asclaimed in claim 26 wherein the central region is spaced from the planeof the disc.
 28. An ultrasonic motor as claimed in claim 26 wherein thecentral region is contained within the plane of the disc.